1. Field of the Invention
The field of the invention is atmospheric pressure mass spectrometry (MS), and more specifically a process and apparatus which combine infrared laser ablation with electrospray ionization (ESI) to provide three-dimensional molecular imaging of chemicals in specimens, for example, metabolites in live tissues or cells.
2. Description of the Related Art
Three-dimensional (3D) tissue or cell imaging of molecular distributions offers insight into the correlation between biochemical processes and the spatial organization of cells in a tissue. Presently available methods generally rely on the interaction of electromagnetic radiation (e.g., magnetic resonance imaging and fluorescence or multiphoton microscopy) or particles (e.g., secondary ion mass spectrometry, SIMS) with the specimen. For example, coherent anti-Stokes Raman scattering provides exquisite lateral and depth resolution for in vivo imaging of lipid distributions on cellular or subcellular level. They, however, typically report on only a few species and often require the introduction of molecular labels. These obstacles are less pronounced in methods based on mass spectrometry (MS) that report the distributions for diverse molecular species. Imaging by SIMS and matrix-assisted laser desorption ionization (MALDI) are appealing because they capture the two- and three-dimensional distributions of endogenous and drug molecules in tissue and whole-body sections. Characteristic to these methods is the requirement for delicate chemical and physical sample manipulation and the need to perform the imaging experiment in vacuum, preventing the study of live specimens.
Ambient MS circumvents these limitations by bringing the ionization step into the atmosphere while minimizing chemical and physical treatment to the sample. During the past few years, this field has experienced rapid development providing us with an array of ambient ion sources. Desorption electrospray ionization (DESI) in combination with MS has been successful in various applications, including the detection of drugs, metabolites and explosives on human fingers, and the profiling of untreated bacteria. Most recently, DESI and extractive electrospray ionization have been used in metabolomic fingerprinting of bacteria. In atmospheric pressure (AP) IR-MALDI and in MALDESI, a combination of MALDI and DESI, the energy necessary for the desorption and ionization of the analyte is deposited by a mid-IR and a UV laser, respectively. In electrospray laser desorption ionization (ELDI) the efficiency of ion production by a UV laser is enhanced by postionization using an electrospray source.
Laser ablation electrospray ionization (LAESI) is an ambient technique for samples with high water content, e.g., cells, biological tissues, aqueous solutions or wetted surfaces. A laser pulse at ˜2.9 μm wavelength ablates a minute volume of the sample to eject fine neutral particles and/or molecules. This laser plume is intercepted by an electrospray and the ablated material is efficiently ionized to produce mass spectra similar to direct electrospray ionization. With LAESI we have demonstrated metabolic analysis of less than 100 ng tissue material from volumes below 100 pL. As in LAESI the laser energy is absorbed by the native water in the sample, the photochemical damage of the biologically relevant molecules, such as DNA, peptides, proteins and metabolites is negligible.
Ambient imaging mass spectrometry (IMS) captures the spatial distribution of chemicals with molecular specificity. Unlike optical imaging methods, IMS does not require color or fluorescent labels for successful operation. A handful of MS-based techniques has demonstrated molecular two dimensional (2D) imaging in AP environment: AP IR-MALDI and DESI captured metabolite transport in plant vasculature and imaged drug metabolite distributions in thin tissue sections, respectively. Recently, through 2D imaging LAESI provided insight into metabolic differences between the differently colored sectors of variegated plants. The lateral resolution of these methods generally ranged from 100 to 300 μm. For AP MALDI and LAESI, improved focusing of the incident laser beam, oversampling, and the use of sharpened optical fibers for ablation could offer further advances in spatial resolution, whereas for DESI imaging, decreased solution supply rates, smaller emitter sizes and the proper selection of the nebulizing gas velocity and scan direction were found beneficial.
Post mortem tissue degradation and loss of spatial integrity during sample preparation are serious concerns in the investigation of biological systems. Cryomicrotoming and freeze-fracture techniques generally practiced in IMS experiments aim to minimize chemical changes during and after tissue and cell preparations. Further complications may arise due to analyte migration in the matrix coating step of MALDI experiments. In vivo analyses circumvent these problems by probing the chemistry of samples in situ. For example, LAESI mass spectrometry reveals the tissue metabolite composition within the timeframe of a few seconds. Instantaneous analysis and no requirement for sample preparation make this approach promising for in vivo studies.
Volume distributions of molecules in organisms are of interest in molecular and cell biology. Recently LAESI MS showed initial success in depth profiling of metabolites in live plant tissues but 3D imaging is not yet available for the ambient environment.